This application pertains to the art of intrinsically safe or protection barrier devices, and more particular to provision of such barrier devices which function in a non-destructive manner.
The application is particularly applicable to provision of electric power to devices disposed within hazardous areas, and will be described with particular reference thereto. It will be appreciated, however, that the invention has broader application, such as in any application in which a device is to be isolated against over voltages, over current, reverse polarity connections, and the like.
Intrinsic safety is generally considered the safest method of supplying electricity to equipment disposed in hazardous areas. Such hazardous areas include highly combustible areas, and include such applications as monitoring devices disposed in fuel storage areas. Such intrinsic barrier devices provide a reliable, less expensive, and more desirable approach to providing power to systems in such areas then are found in the alternatives. Alternatives include such means as encasing an entire system within an explosion proof barrier. Such barriers are obviously bulky, high in weight, and extremely expensive to construct, and maintain.
Presently, several intrinsically safe barrier devices are available. These include "zener barrier modules" which typically include zener diodes, a resistor, and a fuse. The zener diode functions to clamp over voltages. In the event of excessive current or over voltage conditions, the fuse will blow. The blowing of a fuse in such a barrier module typically requires replacement of the entire subsystem, given that a substantial possibility exists that the zener diode, or diodes, were damaged by the clamping action. Such replacement is generally expensive. In addition, there is generally no means to readily discern whether, in fact, a fuse in such a zener barrier module has been blown.
Other attempts have been made to fabricate alternative secure intrinsically barrier devices, typically with systems utilizing a plurality of electronic components. As electronic components themselves require power for operation, other disadvantages arise. In particular, a separate power supply must be provided to the device, or the device is subject to voltage drops between its input and output. Such voltage drops are generally unacceptable, especially in areas where the voltage supplied to the hazardous area must be maintained in close tolerance to generated voltage levels.
The present invention contemplates a new and improved intrinsically safe barrier device which overcomes all of the above-referred problems, and others, and provides a reusable barrier device which is reliable and economical.
In accordance with the present invention, there is provided an protection barrier device which includes first and second input terminals adapted for connection of the device with an associated direct current voltage source. A first series path is provided between the first input terminal and first output terminal. Similarly, a second series path is provided between the second input terminal and second output terminal. A current sensing means is provided to sensing current levels in at least one of the first and second series paths. Means is provided for generating a signal representative of an event in which excessive currents, defined to selected standards, causing a triggering of a crowbar circuit disposed between the first and second series paths. This crowbar circuit, in turn, provides a low resistance current path between the series paths. Current flowing through this crowbar circuit path, in turn, triggers a switching means which functions to impede current flow through at least one of the first and second series paths.
In accordance with a more limited aspect of the present invention, means is also provided for protecting the hazardous area from receiving significant voltage when the first and second input terminals are misswired to reversed potentials.
In conjunction with a yet more limited aspect of the subject invention, a thermistor is disposed in at least one of the first and second series paths, which thermistor functions to minimize current in the event the switching means is bypassed, or fails.
In accordance with another aspect of the present invention, a method of in&rinsic protection for operation of the afore-mentioned means is disclosed.
An advantage of the present invention is the provision of a system which reacts quickly to over current conditions, over voltage conditions, reverse polarity conditions, and short circuit conditions.
Another advantage of the present invention is the provision of a system having a readily discernable visual indicator of when a fault has been detected, and compensated for.
Yet another advantage of the present invention is the provision of a system for intrinsically safe voltage protection which maintains a minimum voltage drop between input and outputs thereof.
Further advantages will be obvious to one of ordinary skill in the art upon a reading and understanding of the subject invention.